Question

I have a MySQL Left Join problem.

I have three tables which I’m trying to join.

A person table:

CREATE TABLE person (     id INT NOT NULL AUTO_INCREMENT,     type ENUM('student', 'staff', 'guardian') NOT NULL,     first_name CHAR(30) NOT NULL,     last_name CHAR(30) NOT NULL,     gender ENUM('m', 'f') NOT NULL,     dob VARCHAR(30) NOT NULL,     PRIMARY KEY (id) );

A student table:

CREATE TABLE student (     id INT NOT NULL AUTO_INCREMENT,     person_id INT NOT NULL,     primary_guardian INT NOT NULL,     secondary_guardian INT,     join_date VARCHAR(30) NOT NULL,       status ENUM('current', 'graduated', 'expelled', 'other') NOT NULL,     tutor_group VARCHAR(30) NOT NULL,     year_group VARCHAR(30) NOT NULL,     PRIMARY KEY (id),     FOREIGN KEY (person_id) REFERENCES person(id) ON DELETE CASCADE,     FOREIGN KEY (primary_guardian) REFERENCES guardian(id),     FOREIGN KEY (secondary_guardian) REFERENCES guardian(id),     FOREIGN KEY (tutor_group) REFERENCES tutor_group(name),     FOREIGN KEY (year_group) REFERENCES year_group(name) );

And an incident table:

CREATE TABLE incident (     id INT NOT NULL AUTO_INCREMENT,     student INT NOT NULL,     staff INT NOT NULL,     guardian INT NOT NULL,     sent_home BOOLEAN NOT NULL,     illness_type VARCHAR(255) NOT NULL,     action_taken VARCHAR(255) NOT NULL,     incident_date DATETIME NOT NULL,     PRIMARY KEY (id),     FOREIGN KEY (student) REFERENCES student(id),     FOREIGN KEY (staff) REFERENCES staff(id),     FOREIGN KEY (guardian) REFERENCES guardian(id) );

What I’m trying to select is the first name, last name and the number of incidents for each student in year 9.

Here’s my best attempt at the query:

SELECT p.first_name, p.last_name, COUNT(i.student) FROM person p, student s  LEFT JOIN incident i ON s.id = i.student  WHERE p.id = s.person_id AND s.year_group LIKE '%Year 9%';

However, it ignores any students without an incident which is not what I want – they should be displayed but with a count of 0. If I remove the left join and the count then I get all the students as I would expect.

I’ve probably misunderstood left join but I thought it was supposed to do, essentially what I’m trying to do?

Thanks for your help,

Adam

Answer 1

What you are doing is fine, you just missed off the group by clause

SELECT p.first_name, p.last_name, COUNT(i.student) FROM person p, student s  LEFT JOIN incident i ON s.id = i.student  WHERE p.id = s.person_id AND s.year_group LIKE '%Year 9%' GROUP BY p.first_name, p.last_name; 

Here’s some test data

insert into person values(1, 'student', 'Alice', 'Foo', 'f','1970-01-01'); insert into person values(2, 'student', 'Bob', 'Bar', 'm','1970-01-01');  insert into student values(1,1,0,0,'', 'current','','Year 9'); insert into student values(2,2,0,0,'', 'current','','Year 9');  insert into incident values(1,1,0,0,0,'flu','chicken soup', '2008-01-08'); 

And here’s the output of the query with the group by added to it:

+------------+-----------+------------------+ | first_name | last_name | COUNT(i.student) | +------------+-----------+------------------+ | Alice      | Foo       |                1 | | Bob        | Bar       |                0 | +------------+-----------+------------------+ 

You could further clean up the query by making join clauses from your where clause, and grouping on the person id:

SELECT p.first_name, p.last_name, COUNT(i.student) FROM person p INNER JOIN student s ON(p.id = s.person_id) LEFT JOIN incident i ON(s.id = i.student) WHERE s.year_group LIKE '%Year 9%' GROUP BY p.id;